Fixes issue https://github.com/magento/magento2/issues/22407. Patch originating from commit https://github.com/magento/magento2/commit/bd5b7c584de95da97200be47d6f63fd9eda372f7.

MAGENTO_22407.patch

--- a/Plugin/Block/Topmenu.php
+++ b/Plugin/Block/Topmenu.php
@@ -193,4 +193,21 @@ protected function getCategoryTree($storeId, $rootId)
 
         return $collection;
     }
+
+    /**
+     * Add active
+     *
+     * @param \Magento\Theme\Block\Html\Topmenu $subject
+     * @param string[] $result
+     * @return string[]
+     */
+    public function afterGetCacheKeyInfo(\Magento\Theme\Block\Html\Topmenu $subject, array $result)
+    {
+        $activeCategory = $this->getCurrentCategory();
+        if ($activeCategory) {
+            $result[] = Category::CACHE_TAG . '_' . $activeCategory->getId();
+        }
+
+        return $result;
+    }
 }